Foams of high temperature processing thermoplastic resins such as, for example, polystyrene, polyethylene, polycarbonate are useful industrial products because of their excellent heat-insulating, cushioning, and other properties. Accordingly, such foams are widely used for various purposes in various market fields associated with soundproof, insulating materials, in particular thermal insulation, cushioning and vibration-isolating materials, light reflection plates, light diffusion plates etc. and as raw materials for the fabrication of various shaped articles.
The foaming of high temperature processing thermoplastic resins by using chemical foaming agents is well known in the art.
A chemical foaming agent decomposes on heating to a specific temperature to yield a vapor or gas or mixture of vapors and gases. The chemical foaming agent is typically incorporated in the thermoplastic resin at a temperature below the decomposition temperature of said foaming agent. The mixture so obtained is then subsequently heated to a temperature above the foaming agent's decomposition temperature whereupon the chemical foaming agent decomposes to liberate a gas or vapor which forms small voids within the thermoplastic resin.
Tetrazole compounds have been widely used as chemical foaming agent for foaming high temperature processing thermoplastic polymers.
The use of the commercially available 5-phenyltetrazole in the foaming of thermoplastic materials is notably described in U.S. Pat. No. 3,442,829.
While 5-phenyltetrazole is an effective high temperature foaming agent having a decomposition temperature in the range of about 230° C. to 250° C., it has been found to have deficiencies in certain applications. For example, it tends to cause discoloration when it is not completely decomposed. When 5-phenyltetrazole is used with polycarbonate resins, some polymer degradation takes place during the foaming process as evidenced, for example, by a reduction in foam impact strength.
U.S. Pat. No. 4,774,266 discloses the use of N-substituted 5-phenyltetrazole compounds as chemical foaming agents for the foaming polymers and copolymers having high processing temperatures. It was especially noted that these type of chemical foaming agent minimize polymer degradation and provide products with increased impact strengths.
U.S. Pat. No. 3,873,477 describes tetrazole metal salts of zinc, barium, calcium, lead and aluminum and their use a chemical foaming agent in high temperature processing, in particular polycarbonate.
Bis-tetrazoles wherein the two tetrazole rings are bridged by a direct bond or a divalent organic residue, in particular aliphatic or araliphatic residues, in 5,5′-position or in 1,1′-position have been described in U.S. Pat. No. 4,142,029 as useful chemical foaming agents for thermoplastic resins. They decompose at temperatures of about 230° to 300° C.
Aromatic poly(aryl ether sulfone) polymers, such as notably polyphenylsulfone (PPSU) polymers, polyethersulfone (PESU) polymers, polysulfone (PSU) polymers are also known high temperature processing thermoplastic and possess a number of attractive features such as excellent high temperature resistance, good electrical properties, toughness and tolerance to a host of chemical environments.
Polysulfone (PSU) polymers are commercially available from Solvay Specialty Polymers USA, L.L.C. (i.e., under the trademark UDEC). PSU has a high glass transition temperature (e.g., about 185° C.) and exhibits high strength (>70 MPa) and toughness (>65 J/m notched impact, 50-100% elongation at break).
Polyphenylsulfone (PPSU) polymers are recognized for instance, for their high temperature capabilities (with a glass transition temperature (Tg) of 220° C.), for its outstanding chemical resistance and for its inherent flame retardant properties.
RADEL® R is for example a polyphenylsulfone (PPSU) polymer commercially available from SOLVAY SPECIALTY POLYMERS USA, LLC.
Polyethersulfone (PESU) polymers are also known for their high temperature capabilities, also having a glass transition temperature (Tg) of about 220° C. These polyethersulfone (PESU) polymers have a high heat resistance, hydrolysis resistance in steam and hot water environments, good overall chemical resistance and excellent electrical properties.
Commercially available (PESU) include notably VERADEL® PESU available from Solvay Specialty Polymers USA, L.L.C.
It is generally known that the decomposition temperature of the chemical foaming agent should ideally be about 20° C. below the maximum foaming temperature in order to obtain a homogeneous cellular structure and maximum utilisation of the chemical foaming agent.
It is, of course, of great importance that the chemical foaming agent be homogeneously dispersed within the thermoplastic material to obtain a product having a uniform cellular structure. It is generally known that chemical foaming agents may be present in powder form, concentrates or masterbatches in the form of pellets. Powder foaming agents, such as notably 5-phenyltetrazole, can lead to feeding problems in the foam extruder as powders are more difficult to meter. In general, it is easier to feed a foaming agent in the form of a pelletized foaming agent masterbatch that is made with a carrier material. However, these carrier polymers can influence the foaming process and need to be compatible with the polymer to be foamed.
In view of the above, the choice of the chemical foaming agent and any other additive will be of ultimate importance in the development of foam materials including the aromatic poly(aryl ether sulfone) polymers, such as notably polyphenylsulfone (PPSU) polymers, polyethersulfone (PESU) polymers, polysulfone (PSU) polymers having high Tg values, as detailed above.
There is thus still a need for foamable compositions comprising aromatic poly(aryl ether sulfone) polymers which can provide foam materials having superior mechanical properties such as high stiffness and strength properties, good electrical, and fire performance characteristics, improved thermal performance capabilities, in particular more robust resistance to very high temperatures used in the manufacturing of structural foamed articles and having improved chemical resistance, improved structural integrity, high void content and closed cell structures.